How to say NO to people without hurting them? I learn it the hardest way, which is from own experience that there was no way to do that without hurting them or getting yourself hurt.

Just recently, i turned down a proposal.. then someone turned me down. Well, who said life is NOT fair? Then.. I am about to do that again to someone, soon or, am i? Yeah, why do i kept shutting-off my heart? Should i just wait till my heart recover or give it a chance? Everything happened for a reason, rite? So..is this the reason?